1. Field of the Invention
The present invention refers to a valve assembly for venting diecasting moulds. The valve assembly comprises a venting channel, a venting valve communicating with the venting channel and having an axially displaceable valve closure member, and an actuating member for operating the venting valve from an open position into a closed position.
The actuating member includes a power pick up member exposed to the liquid casting material escaping during the casting operation from a casting chamber provided in the diecasting mould through the venting channel and being axially displaceable under the influence of the kinetic energy transmitted from the liquid casting material to the power pick up member when the liquid casting material hits the power pick up member. The power pick up member is mechanically coupled to the axially displaceable valve closure member and it includes an axially displaceable push member.
2. Prior Art
Valve assemblies of this kind normally are mounted on one of the two portions of a diecasting mould, said two portions contacting each other along a separating plane when the mould is in its initial position ready for a casting operation. Usually, the design is such that one of the mould portions comprises a recess near its edge whereby the valve assembly is mounted in this recess such that its front surface flushes with the above mentioned separating plane; thus, the front surface of the valve assembly sealingly contacts the front surface of the other mould portion when the two mould portions are assembled, i.e. the diecasting mould is closed. The venting channel of the valve assembly is open towards the above mentioned front surface and forms a continuation of the venting channel of the diecasting mould which is, as well, open to said front surface.
The end of the venting channel of the valve assembly leading to the atmosphere is either open or connected to a vacuum pump means. The vacuum pump means thereby serves to remove the air contained in the casting chamber of the mould when the liquid casting material is filled into the mould under high pressure and with high velocity.
Once the venting process has come to an end, the venting valve assembly closes to prevent the liquid casting material from flowing out of the mould or into the vacuum pump connected to the venting channel. The closing of the valve assembly is initiated by means of the liquid casting material flowing into the venting channel of the valve assembly. For this purpose, according to prior art, an impact or dynamic pressure built-up in the interior of the valve assembly under the influence of the liquid casting material flowing into the mould is used to close the venting valve. In view of the fact that the liquid casting material flows into the mould under very high pressure and, after having filled the casting chamber of the mould, flows with the same high pressure and speed into the venting channel of the valve assembly towards the venting valve, the operation of the venting valve must be initiated with a very small time lag in order to ensure that the closing operation of the venting valve has come to an end before the liquid casting material has reached the venting valve. It is of paramount importance that the liquid casting material is prevented from penetrating the interior of the venting valve because, in such a case, the venting valve would be blocked.
Experiences with such valve assemblies have shown that it is possible to realize an operation of the venting valve with a time lag of extremely short duration (i.e. less than 1 msec) if the means for transmitting the above mentioned dynamic or impact pressure to an actuating force of the venting valve are appropriately designed; thus, a reliable operation of such a venting valve can be ensured.
The Swiss Patent Nr. 633,208 discloses a venting valve assembly of the kind mentioned above which is intended to be used in connection with a high pressure diecasting operation and which, generally speaking, operates quite well. In fact, this valve assembly comprises a plunger valve member whose central axis runs perpendicular to the front surface of the valve assembly, including a valve cylinder communicating with the venting channel and a valve piston displaceable in the valve cylinder. The valve piston is provided with a piston head which projects at the front side of the valve assembly into the venting channel if the valve assembly is open, and which is displaced into the interior of the valve cylinder if the valve assembly is closed.
Further, there is provided an actuating assembly including a power pick up member in the form of a piston-cylinder-arrangement having an operating cylinder communication with a portion of the venting channel which lies in front of the valve cylinder as seen in the direction of flow of the liquid casting material. The operating cylinder comprises a piston member which is exposed to the liquid casting material flowing into the venting channel. The operating cylinder of the actuating assembly and the valve cylinder of the plunger valve member are located in parallel axial relationship, and the piston member of the actuating assembly is connected to the valve piston such that the piston member of the actuating assembly, once having been exposed to the liquid casting material and accelerated to a backward motion, effects a displacement of the valve piston in the sense of a closing of the venting valve.
In the design according to the above mentioned Swiss Patent Nr. 633,208, the piston member of the actuating assembly is connected to the valve piston either directly or via a driver member whereby said driver member is displaced along a path running parallel with the path of motion of the valve piston. According to a variant of this design, it is provided that the piston member of the actuating member is non-positively coupled to the driver member. In any case, however, the path along which the piston member of the actuating assembly is displaced is the same as the path of the valve piston; in practice, this path amounts to 5-10 mm. This circumstance has proven to be disadvantageous in several respects.
Under the impact or shock of the liquid casting material hitting the piston member of the actuating assembly, the mass set in motion (i.e. the piston member of the actuating assembly, the driver member and the valve piston) includes quite a high amount of kinetic energy at the end of the path of motion which must be absorbed at the end of the path of motion by decelerating the moved mass. A rigid stop member probably could lead to a deterioration of the whole valve assembly. Thus, it is required that suitable shock absorbing means be provided which, in turn, require a high expenditure, especially if the time or the path during which the mass has to be decelerated is relatively short. In this connection, it must be pointed out that the plunger piston of the venting valve has a well defined path to run until it is in its predefined closing position. If this path of displacement is increased during the process of deceleration, even during a very short period, the liquid casting material can penetrate the valve cylinder with the result that the operation of the venting valve is severely disturbed or even made impossible.
On the other hand, during the operation of the piston member of the actuating assembly, a portion of the cylinder wall of the actuating cylinder is exposed to the liquid casting material. This freely exposed portion of the cylinder wall has the same length as the operating stroke of the piston member. Thus, the liquid casting material penetrating the actuating cylinder forms, after its solidification, a plug which can be jammed in the actuating cylinder. Due to its length, its removal out of the cylinder bore is difficult and requires a great force. The result is that the two part mould cannot be opened easily and that the surface of the cylinder of the actuating assembly can be damaged. In any case, a rapid wear of the cylinder bore surface must be take into account, particularly if the liquid casting material is of aggressive nature.